Need explanation how to remove small square patterns on the surface

[Urban]

Hi Esben,
I did just test to render the original file.

It worked without any error. So we can say that this problem is solved now 

Thanks again!

It still render so too early at this moment to check the ground plane interfierence. But thats propably the case.

And finally thanks all for your quick replies!
All the best,
/Urban